1. Field of the Invention
The present invention relates to apparatus for transmitting signals between relatively rotatable members and, more specifically, to a slip ring type assembly for transmitting high frequency light signals representing digital information between such members without physical contact.
2. Description of the Prior Art
It is often desirable to transmit information to and from a rotatable member as, for example, a rate table upon which various pieces of equipment such as gyroscopes and the like are being tested, and a non-rotating member such as the surrounding environment where Power Sources and Information Handling Equipment are located. In the past, it has been most common to use mechanical type slip rings which include a slider moving over a surface so that electrical signals may pass therebetween. Such devices suffer a number of disadvantages including jittering of the wiper, which produces noise, and the creation of dust particles by the frictional engagement of the two members, which requires periodic cleaning. The prior art is also able to handle signals only at very low speeds. To overcome these problems, it has been proposed to use optical transfer of information by utilizing an optical light source on one of the two members and an optical detector on the other of the two members. For example, in the Robert W. Upton, Jr. U.S. Pat. No. 4,190,318 issued Feb. 26, 1980, and assigned to the assignee of the present invention, an optical slip ring is disclosed in which radial light signals formed in a continuous ring are transmitted between a barrel and a surrounding sleeve within which the barrel is rotatably mounted. The ring of light signals is produced and detected respectively by a plurality of light emitting diodes and light detector means carried on the barrel and the sleeve. The light detector means may include an optical fiber bundle for transmitting light signals to a remote detector. A difficulty encountered with apparatus of this type is that the light emitter and light detectors are spaced at various distances from each other during the process of rotation. Because of this, the transmission time from the transmitter to the detector is greater when the distances are large than when the distances are small. As a result, the output of the detector has a limited bandwidth and high frequency signals cannot be transmitted across the gap. The ability to handle high frequency signals is important to increase the speed of transmission for digital type information.
Some prior art devices have transmitted information optically in the axial direction and using a larger number of detectors as in the Tewsley U.S. Pat. No. 4,055,058 or a large number of fiber optic members in a bundle as in the Iverson U.S. Pat. No. 4,109,998. With axial transmissions, a higher frequency is available but a difficulty is encountered in that the number of independent separate transmissions is quite limited. For example in the Tewsley patent only two concentric rows of transmitters are shown and in the Iverson, only three concentric fiber optic bundles are available. It is desirable to be able to transmit information between the rotating and non-rotating members along an axis which is perpendicular to the axis of rotation as was done in the above-mentioned Upton patent because a large number of individual channels can then be stacked one on top of another along the axial direction.
Another problem encountered in the prior art is the difficulty in handling high frequency signals because of the angle of spread from the transmitter to the detectors. When the spread of the beam is large, as in the Upton patent mentioned above, the paths of some beams is greater in length than others. This produces a "mushy" signal where the crisp edges of a square wave, for example, become rounded and the space between two consecutive pulses fills in. Without increasing the space between pulses (which decreases the frequency) the separate pulses become less distinct and may even be obliterated. As a result, frequencies in excess of about 8 megahertz cannot be handled.